PLA corner and end excess

I have two questions about printing with PLA. They may or may not be related.

  1. At the beginning and end of every extrusion path, a bit of excess material gets printed. Here is an example of a C-shaped part with the random seam position selected.
    OozeBlobs.JPG
    The surface is supposed to be smooth. Is there a way to adjust the slic3r settings to change this? I have tried changing the retraction value, with no noticeable effect.

Selecting the aligned seam option results in so much excess material printing that it builds up in the Z direction, so tall and narrow parts like this one get knocked off the print bed.

  1. Every external corner prints excess material, so the corners are oversize. Here is an example.
    Corners.JPG
    This is a print of http://www.thingiverse.com/thing:38418. The external surfaces are flat in the .stl file, but are clearly not in this print. Although the look of this particular print was improved by oversize corners, I would like to learn a way to adjust this effect. Is there an adjustment for this?

Try increasing the retraction speed (+2) or length(by 0.1mm) slightly. Also make sure that you are printing with your fan on, as PLA loves to be actively cooled.